|Subject: bees and physics
|is it true that according to
modern physics, a bumble bee shouldn't be able to fly? |
this about ten years ago, and its kept me from having much faith in any
science i've been taught.
|Subject: re: bees and
|No it is not true. Study of the
bumblebee's wings shows that the movement and flexion of the wings
generates enough lift for the bee to fly. It was thought, a long time ago,
that the wings were too small for the body size but they are demonstrably
perfectly adequate for the job - if they weren't, the bee wouldn't
I have heard creationists using the old "bumblebee
hypothesis". Their trouble is that once they get an idea that seems to
support their proposal, they hang onto it even though it has been
Observation - the bumblebee flies using its wings to
Hypothesis 1 - the wings are too small for the bee to fly.
This disagrees with the observation so must be wrong
If Hypothesis 1 is
wrong it must be rejected. The bee obviously flies, therefore there is
something happening that increases the apparent lift of the wings. It is
then discovered that the flexibility of the wing plays a significant role
as well as the mechanical movement of the wing through the air and the
speed at which the wing is flapped.
Science does not stop at
hypothesis 1 but rejects it and goes on to discover what is really
|Subject: re: bees and
|Here's the relevant text from the
last link, formatted to make it more readable:|
It is obvious to any scientist that the bumble bee can
fly as experiment proves it. So what is this business about proving bees
cannot fly? And who started it?
First let's look at the physics
behind the story. If you are asked about flight the first thing you do is
to use the equations which describe how much lift an object has. You
compare the lift to the weight of the object. If the lift is greater than
weight then the thing can fly. Bumble bees are pretty big, weighing almost
a gram, and have a wing area of about a square centimetre. Tot up all the
figures and you find that it cannot generate enough lift at its typical
flying speed of about one meter per second. But that doesn't prove bees
cannot fly. It proves that bees with smooth, rigid wings cannot glide.
Experiment has proven this too. With the aid of dead bees and a little
lacquer it is easy to show that they really cannot glide.
So how do
they fly? Actually that turns out to be a very interesting question and
one that reveals a lot of physics. Why do bees flap while jumbo jets have
fixed wings? It is a question of size and this is revealed in a figure
called the Reynolds Number. Osborne Reynolds was a Victorian engineer who
was interested in what happens when you place an object in a stream of
liquid or gas. The number named after him is a ratio which tells us, for a
particular object, how much lift you get compared to how much drag or
resistance you get. A low Reynolds number means little lift for a lot of
drag and a large Reynolds number means a lot of lift.
Number depends on the size of the wing. Bigger wings give bigger Reynolds
numbers. Now if, again, you put in all the numbers you find that bees work
at very low Reynolds Numbers (1000 or so for a honeybee, as little as 15
for the aphid-eating chalcid wasp). This means that their flight is very
inefficient because as a wing starts to move to create lift the drag holds
it back. It is fairly straightforward to show that birds can generate
enough lift to fly once they are in motion with air flowing smoothly over
their wings, but many of them would have great difficulty taking off.
Small insects, according to this model, cannot fly at all. Of course, all
this proves is that the model is incomplete.
Some brilliant work
by Torkel Weis-Fogh has shown us how small insects do fly and it has led
to some rather neat insights into nature's cunning. If you are small and
want to fly you have a problem. The Reynolds Number is against you so you
cannot glide and flapping is very hard work. A wing is a device which
encourages the air to flow over it so that when it leaves the rear wing
edge, the air moves downwards. That produces a thrust upwards on the wing.
A smoke-filled wind tunnel shows this beautifully with curling eddies of
smoke flicking off the wing edges. Unfortunately to make a good eddy takes
time. The wing has to move a few times its own length to get things
started. This makes it tricky if you are going to flap as the maximum
travel of a wing is about its own length and very little lift is generated
for most of the stroke. Nature has come up with a number of interesting
solutions to this problem of which the "clap-fling" is a good example.
When a small bird or insect wants to take off it needs a lot of lift. What
it does is bring its wings together above its back so they clap, expelling
air from between them. As the wings are separated, air is drawn quickly in
to fill the void. The wings are flung apart and lift is generated
immediately as the air is already in motion in the correct way. You can
hear the clap. The characteristic whirring of a pheasant taking off is
caused by its wings clapping. Almost 2000 years ago Virgil recorded in The
Aeneid that a rock dove claps its wings as it takes off - a passage he
stole from Homer but he added the bit about the clapping.
asking how bees fly we find that they are remarkably clever about it.
Aircraft can generate enough lift that they do not need such tricks, but
they do need long runways. Birds get enough lift to fly but for take-off
need a boost. Just the poor old bee and about a million different species
of winged insect need some extra trickery to stay aloft.
did it all start? Where does the story date back to? J.H.Mcmasters states
that the story was prevalent in the German technical universities in the
1930's, starting with the students of the aerodynamicist Ludwig Prandtl at
Göttingen. The story he tells is that a noted Swiss aerodynamicist, whom
he does not name, was talking to a biologist at dinner. The biologist
asked about the flight of bees and the Swiss gentleman did a
back-of-the-napkin calculation of the kind I described. Assume a rigid,
smooth wing and so on. Of course, he found that there was insufficient
lift and went away to find out the correct answer. In the
meantime the biologist put the word around, presumably to show that nature
was greater than engineering, and the media picked it up. The truth, as
now, wasn't newsworthy so a correction has never been publicised. The man
on the Clapham omnibus, therefore, continues to tell me that science is a
load of crock because it once proved that bumble bees cannot
İKen Zetie, Clarendon Laboratory, University of
|From: Karl Kruszelnicki
|Subject: re: bees and
What a great answer, and such
a neat little story at the
This forum is un-moderated. The views and opinions expressed are those of the individual
poster and not the ABC. The ABC reserves the right to remove offensive or